Java HashMap

Java HashMap

A HashMap stores items in key/value pairs, where each key maps to a specific value.

It is part of the java.util package and implements the Map interface.

Instead of accessing elements by an index (like with ArrayList), you use a key to retrieve its associated value.

A HashMap can store many different combinations, such as:

• String keys and Integer values
• String keys and String values

Create a HashMap

Create a HashMap object called capitalCities that will store String keys and String values:

import java.util.HashMap; // Import the HashMap class

HashMap<String, String> capitalCities = new HashMap<>();

Now you can use methods like put() to add key/value pairs, get() to retrieve a value by key, and remove() to delete an entry - all by using keys instead of index numbers.

Add Items

To add items to a HashMap, use the put() method:

// Import the HashMap class
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap object called capitalCities
    HashMap<String, String> capitalCities = new HashMap<String, String>();

    // Add keys and values (Country, City)
    capitalCities.put("England", "London");
    capitalCities.put("India", "New Dehli");
    capitalCities.put("Austria", "Wien");
    capitalCities.put("Norway", "Oslo");
    capitalCities.put("Norway", "Oslo"); // Duplicate
    capitalCities.put("USA", "Washington DC");

    System.out.println(capitalCities);
  }
}

Note: In the example above, if the same key (like "Norway") is added more than once, the latest value will overwrite the previous one, because keys in a HashMap must be unique.

Access an Item

To access a value in the HashMap, use the get() method and refer to its key:

capitalCities.get("England");

Remove an Item

To remove an item, use the remove() method and refer to the key:

capitalCities.remove("England");

To remove all items, use the clear() method:

capitalCities.clear();

HashMap Size

To find out how many items there are, use the size() method:

capitalCities.size();

Note: The size only counts unique keys. If a key is added more than once, only the latest value is kept.

Loop Through a HashMap

Loop through the items of a HashMap with a for-each loop.

Note: Use the keySet() method if you only want the keys, and use the values() method if you only want the values:

// Print keys
for (String i : capitalCities.keySet()) {
  System.out.println(i);
}

// Print values
for (String i : capitalCities.values()) {
  System.out.println(i);
}

// Print keys and values
for (String i : capitalCities.keySet()) {
  System.out.println("key: " + i + " value: " + capitalCities.get(i));
}

Other Types

Keys and values in a HashMap are actually objects. In the examples above, we used objects of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:

// Import the HashMap class
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {

    // Create a HashMap object called people
    HashMap<String, Integer> people = new HashMap<String, Integer>();

    // Add keys and values (Name, Age)
    people.put("John", 32);
    people.put("Steve", 30);
    people.put("Angie", 33);

    for (String i : people.keySet()) {
      System.out.println("key: " + i + " value: " + people.get(i));
    }
  }
}

When Order Matters

In the next chapter, you will learn about TreeMap, which stores key/value pairs in sorted order by key.